Altitude for main deployment on L3 flight?

I am hoping to get some expert input, as to what altitude I should depoloy my main parachute. Here are the details:

• The rocket is about 50 lbs, 6" dia., 11' tall, and about 90' from top of main chute to bottom of booster when all harnesses are fully stretched out.
  
• I will be launching in West Palm Beach, Florida. We have a nice clear launch field, but it is bordered by sugar cane crops and alligators, so I don't want to drift too far!
  
• My main is a spherachutes 168" (approx. 9' diameter)
  
• I am using a deployment bag with spherachutes 36" pilot (approx 24" dia.)

If I had all the room in the world to drift, I would go 1,000 or 1,500 ft. But since space is a little limited, I was thinking 800'. What are your feelings?

Thanks,
Cris

Cris

I'm currently building my L3 and also plan to launch at West Palm. The deployment bag is great idea (one I'm still contemplating). However, using one increases the time it will take for your main to fully inflate. A large chute will take time to inflate so it depends on your fall rate under drogue. The last thing you want is late deployment.

My anticipated altitudes for main is 1200 ft for primary altimeter and 1000 ' for the backup. You could drop to 1000' and 800' for backup, but I would advise against anything lower than these settings for a L3 project. You want to be sure the chute has plenty of time to inflate.

It sounds a little low, How long are your shock cords?
The longer the cordage the longer it will take to fully deploy.
If the chute hangs up on anything it will take even longer...............

For my level 3 I went with 1400ft & a back up of 1200ft
It was a 6" dia. 12 ft around 55lbs loaded. I was still able to put it back on the field & was within a 1/4 mile from the launch pad ( it went just shy of 6k)

Back about 10-12 years ago when altimeters were starting to become the norm. people used 250ft for loc Magnums & I have one video from LDRS that it fully deployed just as the booster hit the ground.



JD

Speaking of descent rate under drogue...

My drogue is a 28" flat chute, and I will have about 45' of harness between the two body sections as the rocket falls.

I've been told that this 28" chute will provide sufficient drag. However, rocksim says that my descent rate will be about 150'/sec! Thats about 100mph, right??

Can this be correct? I dont believe that rocksim is taking into consideration that the rocket is broken into two tumbling sections, and that there is a 45' tether between them.

I didnt expect rocksim to be exact on the descent rate, but this is MUCH higher than I expected! Any ideas what my actual descent rate will be, or should I buy a larger drogue?

Cris

I can't comment on determining your exact fall rate, but you are most likely right that Rocksim is not (or cannot) properly calculate the drag coefficient of the separated body tubes which would greatly reduce your fall rate. The other issue is your flat drogue chute has a much lower Cd than a SkyAngle or hemispherical chute so IMO that is a very small drogue for a 50lb rocket.

Just for a comparison: My L3 weighs about 1/2 of yours (it will be about 32lbs at motor burnout) with similar shock cord lengths and I'm using a Tac-9 24" drogue. You need to upgrade your drogue IMO.

I am using spherachutes for my main and pilot, so how about a spherachutes 36"? Of course, 36" is 1/2 of the circumference (the way spherachutes measures 'em) which translates to almost 24" diameter.

Or would I need to go up to a 48" (approx. 30" dia.)?

Cris

I use a 24" TAC drogue from Giant Leap for a LOC Bruiser that weighs about 60 pounds on the way down and a similar sized Skyangle drogue for a rocket that is about 45 pounds. Both work just fine.

Keep in mind that the purpose of the drouge is not so much to slow your rocket down during it's descent but to make sure the rocket falls in the proper orientation so the main can deploy correctly.

Think about all the flights you've seen that were drogueless. Did those come out alright? Some rockets fall better without a drogue.

Your 24" chute should work just fine.

As far as the deployment altitude of the main, the higher the better. It gives the chute more time to deploy and it will also give you a better opportunity to spot it on the way down.

Good Luck on the flight, I hope it goes well.

Andrew Grippo

This one is wide open for interpretation, I prefer to open at 1000 and 900,back up.

One of the best, most experienced fliers I know, flies L-3 rockets on a regular basis, on one altimeter, and deploys at 300 or 400 ft[can't remember which] When I asked him why....."why walk?"

So there ya go!

Cris

The best way to determine this....ask your TAPs! They are who you really have to answer to. Both of my TAPs require me to submit a detailed flight profile. They may leave certain decisions up to you but will advise you appropriately and, perhaps, tell you the way you must do it.

BTW....when's your anticipated flight date?

I think what I'll do then is get a slightly better drogue, in the spherachutes 36" (24" dia.). Its no bigger than my current chute but its a hemispherical design rather than a flat.

As far as the main deployment, I think I'll go with 1000 or 1200. What is the point in staggering the altitudes as many seem to do? Is that so you can spot an altimeter failure?

Cris

Cris:
I'm doing My L3 Sept 13th, Kit is 9.5' Tall, 6" Dia and 42#'s on the pad, I'm using (2) 1/2" Kevlar Shock cords 40' long each, Drogue is a 24" TAC-9 while the Main is a TAC-9B, I'm doing the Main at 1000' and backup at 900'. I've launched this same set up a couple of time's on L's and it all worked great!
Good Luck! -G-

Cris,
I did my L3 with a 33 pound 5.5 dia 12' long rocket less motor. I had 90' of harness between drogue and main. My drogue was a 24" and my Main was a Cert 3 XL. My main was set at 700' with a back up at 400'. A lot will depend on how fast your chute will open up. Staggerd alts is for if your main does not come out completely on the first try the second may push it out and save your flight.

Well, from what i've read, the majority of L3 cert failures are do to a failure in recovery.

So you probably want to give the parachute as much time as you can so that it can full deploy and inflate.

You did say you were surrounded by high crops and man eating alligators, but in all honesty, i'd rather have to jump through some weeds to recovery my l3 cert rocket than have it come down close and too fast due to the chute not opening up. But then again, i'm built like Atlas himself and can easily wrestle a 1000 lbs alligator even on a bad day!

If I had all the room in the world to drift, I would go 1,000 or 1,500 ft. But since space is a little limited, I was thinking 800'. What are your feelings?

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Plus, if space is a little limited, perhaps it may not be the best place to fly your rocket there? Part of flying any rocket is being able to assess whether or not the flying field is the proper size. Not to say it isn't, but definitely something that should be taken into consideration.

SynergyAero said:

As far as the main deployment, I think I'll go with 1000 or 1200. What is the point in staggering the altitudes as many seem to do? Is that so you can spot an altimeter failure?

Cris

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I staggered mine to protect against having both fire at the same time. Last thing I needed was to have 10 grams of BP blow at once when 4 was what I needed and 6 was the back up.

As far as altitude, for your cert I would certainly plan on having deployment be in the 1200-1700' range. Better to err on the side of caution than to risk the rocket touching down before the chute has a chance to inflate. You can always go lower after you get your cert.

Just a few comments.

1.) RS and other parachute calculators don't account for airframe orientation or drag during recovery.

Your rocket, if strung out horizonally without a drogue chute, would have the drag equivalent to a 32" diameter chute weighted with 50 pounds which gives a 98 fps (67 mph) descent rate whereas you rocket coming in ballistically would have less drag than a 6" chute which would be more than 521 fps (355 mph). Reality is somewhere in the middle.

To insure a horizontal attitude after apogee deployment (lower velocity) you can always employ a small drogue chute at either end of your apogee split airframe. Two 28" drogues and a horizontal airframe should produce an apogee descent rate ~60 fps (40 mph).

2.) The safe minimum main parachute deployment altitude depends on several factors: the initial descent speed; the main deployment time and inflation delay, and the density altitude at the ground.

For example if it takes 3 seconds for your main to fully inflate (that's an eternity) in the worst case your rocket will have descended a distance equal to 3 times the initial descent rate. If you rocket is descending at 60 fps, the worst case inflation distance is 180', if the initial descent rate is 100 fps the worst case inflation distance is 300' and if your initial descent rate is 150 fps, the worst case inflation distance is 450'.

I'm not sure whether you main chute diameter are equivalent in drag to a 168" normal chute or corresponds to a 108" real chute, but regardless the main chute descent rates will be 19 fps (13 mph) or 29 fps (20 mph). Certainly you want to be fully slowed down before you run into a tree or building, but once the main is inflated you're down to terminal velocity in a second or two and any more delay is just drift distance.

So worst case we add 3*150' + 2*29' and round to 500' so that's the minimumn reserve distance you need if the chute is going to pop.

If the data is correct the numbers are conservative. If you want to minimize drift I'd be comfortable setting the main deployment for 800' and have your backup go 1 second later. If the main deployment occurs without the backup and is really slow, you're under full main by 300' worst case, and more likely by 500'. If the chute fails to pop at the first charge and makes it out for the second, it still inflates and slows by 150' worst case, and more likely by 350'. If neither charges work, it really doesn't matter cause your rocket's going to crash anyway.

Bob

bobkrech said:

snip...2.) The safe minimum main parachute deployment altitude depends on several factors: the initial descent speed; the main deployment time and inflation delay, and the density altitude at the ground....snipBob

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You forgot the alligators. :surprised:

I have nothing of worth to contribute here.

Thank-you everyone for the very helpful answers! Especially Bob... wow that was a thorough explanation!

Bob confirmed my initial decision to go about 800'. Although to be on the safe side, at least for my certification flight, I'll probably bump it up closer to a thousand. Once I get my cert and am comfortable with the deployment, I'll probably drop it back down to 800.

I contacted Julie at Spherachutes to order a new drogue as well (as you recommended, Mark). Either going to get the 36" (23" dia.) or the 48" (31" dia.).

Thanks again for all the help, and if anyone else has any input, please share! Ill take all the advice I can get...

Cris

When's the cert attempt?

Maybe I'll make it down.

I'm thinking of November of mine, but it's still open.

Also, if your rocket is under drogue and falling at say 70-100 ft/s, then you only got about 8 seconds before the rocket hits the ground.

Be sure you are confident the parachute will fully inflate in less time than that. I've seen quite a bit of launches where the parachutes never fully inflate because the main fires too close to the ground and the rocket smacks pretty hard into the dirt.

Not sure yet, Mark. I have the rocket completed, except for the 'chutes and recovery harness. I have to tie a few knots, and do alot of ground testing. Then a little paint and she's done!

Only problem is I need to set aside about $1100, between the motor case and the reload... so that may take a couple of months!

Probably around the same time as yours. Niki and I will be at the upcoming launches, though in September and October. We'll be launching her Mama Mia! rocket, and maybe some low power stuff I have laying around.

Cris

SynergyAero said:

Only problem is I need to set aside about $1100, between the motor case and the reload... so that may take a couple of months!

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$1100? wow....what motor are you using?

I'm going with an AMW 1350WW. Should hit less than 7500 feet.

Sunday is ground test day, then it's time for paint and my Tap pre-flight inspections

SynergyAero said:

Thank-you everyone for the very helpful answers! Especially Bob... wow that was a thorough explanation!

Bob confirmed my initial decision to go about 800'. Although to be on the safe side, at least for my certification flight, I'll probably bump it up closer to a thousand. Once I get my cert and am comfortable with the deployment, I'll probably drop it back down to 800.

I contacted Julie at Spherachutes to order a new drogue as well (as you recommended, Mark). Either going to get the 36" (23" dia.) or the 48" (31" dia.).

Thanks again for all the help, and if anyone else has any input, please share! Ill take all the advice I can get...

Cris

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One thing to keep in mind concerning your drogue. If it is too large and the rocket is falling too slow the main won't open. It will just blow out of the airframe and flop around all the way to the ground.

I don't have the references right in front of me but there is actually a minimum speed the rocket needs to decend at for the main to have an effective deployment. Off the top of my head I think it is about 50 fps.

Isn't rocket science fun!!!

Andrew

Mark,

I'm using an AMW M1730 SK, and 98-11000 hardware

Cris

Thats a good point you bring up, Andrew, and I will definately take into consideration. Thanks!